1. Field of the Invention
The present invention relates to testing instruments generally and particularly to photoelectric testing instruments for determining stopping times of movable members such as press rams.
2. Description of the Prior Art
Operators of presses with part-revolution clutches are protected by two-hand controls or by presence-sensing devices such as light curtains. When two-hand controls are used, a press STOP signal is initiated whenever the operator releases pressure on one or both press RUN buttons. When a light curtain is used, a press STOP signal is initiated whenever the operator interrupts the light beam protecting the press. The degree of operator protection afforded by two-hand controls and light curtains depends mainly on the speedy performance of the press brake and on the distance the RUN buttons or light curtain is located from the press pinch points such as the ram and die. When an operator initiates a press STOP signal by releasing pressure on one of the RUN buttons or by interrupting the light curtain and moves his hands towards the die, the press brake must stop the press ram before the press operator's hands reach the point of operation or any dangerous press die closure area.
Current OSHA press safety standards require that a minimum safe distance be established between operating buttons and press closure areas when two-hand controls and presence-sensing devices are used. The minimum safe distance for the location of two-hand controls and presence-sensing devices is calculated using the operator's hand speed and the speed with which the press ram is brought to a full stop by the brake after a press STOP signal is initiated. The OSHA standards also require that brake performance -- the ability of the press brake to bring the slide to a full stop within the forementioned time used in estabishing the minimum safe distance -- be checked weekly unless the press has a built-in brake performance monitoring system.
Known devices for performing such required press stop time testing included visually timing the press ram motion after the initiation of a STOP signal. Clearly such a method is highly inaccurate and subjective. Mechanical devices are also known wherein a moving slide is wired into the press control circuitry to initiate and stop a timer to thereby measure stopping times. Such systems require time expenditure to properly connect them to the press circuitry. In such systems there is also the danger that upon removal of the testing system the press circuitry will be left in a dangerous or inoperative condition.